The present invention relates generally to thin metallized and non-metallized polymer films that incorporate additional coatings and surface functionalization, which impart application specific properties such as improved thermal stability, abrasion resistance, moisture barrier and optical effects, that make these films useful in food packaging applications, electrical applications that include film capacitors and cables, metal evaporated magnetic tapes, printing, decorative wraps and optically variable films for security applications.
Metallized and non-metallized films are commonly used in a variety of electrical, packaging and decorative applications. Although the application field is quite broad, the desired properties of the different films are basically the same. These desired properties include mechanical strength, thermal and chemical resistance, abrasion resistance, moisture and oxygen barrier, and surface functionality that aids wetting, adhesion, slippage, etc. As a result, a multitude of hybrid films have been developed to service a wide range of applications.
In general, hybrid metallized and polymer coated films utilize a variety of production methods. For example metallized polymer films are usually corona-, flame-, or plasma-treated, to promote adhesion of the metal to the polymer surface (U.S. Pat. Nos. 5,019,210 and 4,740,385); or ion beam-treated and subsequently electron beam-charged to promote adhesion and flattening of the film onto a substrate by electrostatic force (U.S. Pat. No. 5,087,476). Polymer coatings that serve various functions such as printability, adhesion promotion, abrasion resistance, optical and electrical properties, have been produced using various techniques that include thermal cure, reactive polymerization, plasma polymerization (U.S. Pat. No. 5,322,737), and radiation-curing using ultra-violet and electron beam radiation (U.S. Pat. Nos. 5,374,483; 5,445,871; 4,557,980; 5,225,272; 5,068,145; 4,670,340; 4,720,421; 4,781,965; 4,812,351; 4,67,083; and 5,085,911). In such applications, a monomer material is applied using conventional techniques of roll coating, casting, spraying, etc., and the coating is subsequently polymerized under atmospheric pressure conditions.
More recently, a new technique has been developed that allows the formation of radiation-curable coatings in the vacuum using a flash evaporation technique that leads to the formation of a vapor-deposited thin liquid monomer which can be radiation-cured (U.S. Pat. Nos. 4,842,893; 4,954,371; and 5,032,461 and European Patent Application 0 339 844). This technique overcomes the limitations of conventional techniques for applying the liquid monomers and requires relatively low doses of radiation for polymerization.
The vacuum polymer coating technique as described in the above references was found to have some critical limitations on certain mechanical, thermal, chemical and morphological properties that can reduce their usefulness in packaging films, capacitors, metal evaporated magnetic tapes and optically variable films. The invention disclosed herein overcomes these problems and extends the one-step polymer and metal vacuum coating technique to new functional products with a unique set of properties.
It is an object of the present invention to produce a hybrid polymer film that has superior mechanical, thermal, chemical and surface morphological properties. In conjunction with one or more metal coatings or a ceramic coating, the hybrid film can be used to produce improved packaging films, film capacitors, metal evaporated magnetic tapes and optically variable films.
It is also an object of this invention to produce hybrid films with controlled surface microroughness. This includes films that have a flatter surface than that of the base film, or a surface with controlled microroughness.
It is another object of the invention to provide an improved process for applying, polymerizing, and discharging, one or more layers of vacuum-deposited radiation-curable monomer films that are used to produce the hybrid polymer film in a one-step continuous process.
In accordance with the present invention, a hybrid polymer film comprises a first polymer film having a plasma-treated surface and a second polymer film having first and second surfaces, the first surface of the second polymer film being disposed along the first plasma-treated surface of the first polymer film.
The base, or first polymer, films used in the invention to produce the hybrid films are chosen from a group of thermoplastic films that include polypropylene, polyethylene terephthalate, high and low density polyethylene, polycarbonate, polyethylene-2,6-naphthalate, nylon, polyvinylidene difluoride, polyphenylene oxide, and polyphenylene sulfide, and thermoset films that include cellulose derivatives, polyimide, polyimide benzoxazole, and polybenzoaxozole. The second polymer films are radiation-polymerized monomer films that are multifunctional acrylate or acrylated monomers that contain double bonds capable of radical polymerization. Plasma treatment with gases from the group of N2, Ar, Ne, O2, CO2, and CF4 is used to functionalize the base film, to further improve the cross-linking of the acrylate film surface, and to remove surface charge, which improves winding and unwinding of the hybrid film. Inorganic layers may be used in combination with the polymer layers to produce different end use hybrid films; such inorganic layers include metals selected from the group consisting of aluminum, zinc, nickel, cobalt, iron, iron on aluminum, zinc on silver, zinc on copper, and zinc on aluminum, nickel-cobalt alloys, and nickel-cobalt-iron alloys, and ceramics selected from the group consisting of aluminum oxide, silicon oxides (SiOx, where x=1 to 2), tantalum oxide, aluminum nitride, titanium nitride, silicon nitride, silicon oxynitride, zinc oxide, indium oxide, and indium tin oxide.
The hybrid polymer film evidences both improved corrosion resistance and current carrying ability of metallized capacitors compared to prior art polymer films and overall reliability in demanding applications that require operations in extreme conditions of voltage current and temperature.
As incorporated in food packaging, the presence of the acrylate polymer on a thermoplastic polymer such as polypropylene improves the oxygen and moisture barrier of metallized and ceramic coated films, and it also improves the mechanical properties of the barrier layer to the extent that there is less damage of the barrier layer as a function of film elongation.
By adjusting the chemistry of the acrylate coatings, the surface of the hybrid films can be made hydrophobic/philic, oliophobic/philic and combinations thereof. This can accommodate different printing inks for packaging film applications in addition to the improvement of barrier properties. Such metallized printable film produced in a one-step process eliminates the lamination of an additional polymer film that is used to protect the metal layer and provide a printable surface.
The hybrid films can have reduced surface microroughness, thus eliminating the need for costly flat films for magnetic tape applications. Increased and controlled surface microroughness on a hybrid film can result in lesser abrasion damage and the formation of unique interference effects cause color shifts with changing viewing angle.
As incorporated in electrical flexible cables, fluorinated acrylate polymers deposited on such thermoset polymer films as polyimide, polyimide benzoxazole (PIBO), and polybenzoaxozole (PBO), prevent electrical tracking, and only carbonize in the presence of electrical arcing.
Color shifting effects useful in decorative and security applications can be produced in a one-step low cost process by proper choice of the thickness of the metal and polymer layers.